1. Field
The disclosed method and apparatus relates generally to wireless communications and more particularly to a wireless communications transceiver.
2. Description of the Related Art
Wireless communications devices are being manufactured in increasing numbers for widespread public use. Manufacturers are under increasing pressure to lower cost, and have responded by offering highly integrated transceiver circuits. There is a desire to minimize circuit complexity not only to reduce the size and therefore the cost of the integrated circuit chips, but also to reduce power consumption. Power consumption is especially important for small hand-held devices such as mobile phones. There is also a desire to provide highly integrated circuitry that is capable of operation over two or more frequency bands, such as the respective bands for EGSM (Global System for Mobile Communications), DCS (Digital Cellular Systems), and PCS (Personal Communications Service).
Wireless communications devices typically use digital phase modulation. EGSM, DCS, and PCS in particular may use a minimum frequency-shift keying modulation format having a substantially constant amplitude envelope. Typically the modulated RF (radio frequency) signal is produced from in-phase and quadrature-phase base-band signals. For example, to produce a substantially constant amplitude envelope, the in-phase and quadrature-phase base-band signals are band-limited binary data streams that are offset from each other in time by one-half of a bit period and that are amplitude modulated so that the sum of the squares of the in-phase amplitude and the quadrature-phase amplitude is constant. In practice, the desired in-phase and quadrature-phase base-band signals are digitally synthesized as a function of the data to be transmitted. A pair of digital-to-analog converters convert the digitally synthesized in-phase and quadrature-phase signals to respective analog signals for application to a quadrature modulator capable of producing a modulated RF signal.
Although a quadrature modulator may produce a modulated RF signal directly at the frequency to be transmitted, there are advantages to producing the modulated RF signal at a lower frequency for up-conversion to the frequency to be transmitted. Such a two-step up-conversion process permits the desired performance requirements of the quadrature modulator, such as the tolerable deviation from an ideal amplitude balance and quadrature-phase shift, to be more readily achieved at the lower frequency. For example, it is easy for integrated digital circuitry to produce quadrature-phase carriers at the lower frequency, and the quadrature modulator can be configured as a harmonic rejection mixer in order to reduce spurious effects of the digitally-produced quadrature-phase carriers.
A two-step up-conversion architecture for a wireless transmitter is shown in Kaufman et al. U.S. Pat. No. 6,240,142. The use of a harmonic rejection mixer in this architecture is shown in Weldon et al., “A 1.75 GHz Highly-Integrated Narrow Band CMOS Transmitter with Harmonic-Rejection Mixers,” 2001 IEEE International Solid-State Circuits Conference, Feb. 6, 2001, pp. 160–161, 442. Although these circuits provide an improvement over a direct-conversion transmitter architecture for high levels of integration, there is still a need for decreasing circuit complexity in order to reduce power consumption for hand-held communications devices. The two-step up-conversion architecture of Kaufman et al. uses a multiplicity of high-frequency balanced modulators, including two balanced modulators operating at the RF transmission frequency. The balanced modulators consume a significant amount of power.